Generally, group III-based nitride semiconductors, such as gallium nitride (GaN) and aluminum nitride (AlN), have good thermal stability and a direct transition type energy band structure, and thus have been spotlighted as materials for light emitting devices emitting light in the visible range and in the UV range.
Such group III-based nitride semiconductor layers are grown on a heterogeneous substrate having a similar crystal structure through metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) due to difficulty in fabrication of a homogeneous substrate capable of growing the group III-based nitride semiconductor layers. As the heterogeneous substrate, a sapphire substrate having a hexagonal crystal structure is generally used. However, since sapphire is an electrically non-conductive material, sapphire restricts the structure of a light emitting diode. Accordingly, there have been developed a technique for manufacturing a high efficiency vertical type light emitting diode, in which epitaxial layers such as nitride semiconductor layers are grown on a heterogeneous substrate, such as a sapphire substrate, and a support substrate is bonded to the epitaxial layers, followed by separating the heterogeneous substrate through laser lift-off or the like.
Generally, a vertical type light emitting diode has better current spreading performance than a typical lateral type light emitting diode and exhibits good heat dissipation performance through adoption of a support substrate having higher thermal conductivity than sapphire. Furthermore, a reflective metal layer can be disposed between the support substrate and the semiconductor layers to reflect light traveling towards the support substrate, thereby improving light extraction efficiency.
Further, the vertical type light emitting diode can improve light extraction efficiency through a roughened surface of an epitaxial layer (n-type semiconductor layer) through which light is emitted. To this end, the epitaxial layers are subjected to wet etching such as photo-enhanced chemical (PEC) etching. However, since wet etching can provide damage to a bonding metal layer or a reflective metal layer, there is a need for protection of the bonding metal layer or the reflective metal layer from an etchant.
The vertical type light emitting diode generally employs a conductive support substrate and includes an anode pad on the support substrate and a cathode pad on the epitaxial layers. Furthermore, an electrode extension extending from the cathode pad and electrically contacting the epitaxial layers is used to assist in current spreading within the epitaxial layers. The electrode extension may be formed not only in a central region of the light emitting diode but also near edges of the epitaxial layers to evenly distribute the current over a large area of the epitaxial layers. However, since the cathode pad and the electrode extension are disposed on an epitaxial layer through which light is emitted, light emission is blocked by the cathode pad and the electrode extension, thereby deteriorating luminous efficacy of the light emitting diode.